We previously developed single adenovirus (Ad) vectors that contained the components for a tetracycline-regulatable gene-expression system in the E1 and E3 deletion regions, and showed that the Ad vectors containing the tet-on system exhibit a much inferior regulation of transgene expression than those containing the tet-off system. In many cases, the tet-on system may be preferable because of its positive regulation of transgene expression. To this end, in the present study, by introducing the latest generation reverse tetracycline-responsive transcriptional activator (rtTA2s-M2 or rtTA2s-S2) and the tetracycline-controlled transcriptional silencer (tTS) into the original tet-on system, we constructed various modified Ad-mediated tet-on systems. Among them, the novel single Ad vector, which contained three heterologous gene-expression cassettes of the gene of interest, rtTA2s-S2, and tTS in the E1 deletion region, the E3 deletion region, and the region between E4 and 3'ITR, respectively, displayed vastly improved doxycycline-inducible gene expression in terms of low basal expression, high induced expression, and high responsiveness to doxycycline both in vitro and in vivo. These results also suggest that the low responsiveness to doxycycline may explain why the original tet-on system in the context of the Ad vector is not effective in vivo. This is the first report describing the cloning of three heterologous gene-expression cassettes into three separate regions of the E1/E3-deleted Ad genome. This improved Ad-mediated tet-on system might be useful for gene therapy and greatly facilitate the analyses of gene function.